Illumination systems including an illumination source and a mechanical dimming device for dimming the amount of light generated by the illumination are used in a variety of situations. In particular, if comprising a high intensity illumination source, such illumination systems are widely used e.g. in stage, television, motion picture or architectural lighting systems. For some types of stage presentations, there may be as many as 300 individual illumination sources. Such illumination systems must be modifiable. E.g. in the control of stage or television stage lighting, a sequence of lighting effects are activated in synchronism with the dramatic action on the stage. Potentially as many as 200 lighting effects must be activated in sequence.
In such applications, it is important to ensure that the light intensity of a given light source can be varied between maximum and zero.
With incandescent lamps, dimming is simply facilitated by reducing the input voltage to the lamp. Reducing the voltage causes a temperature drop of the filament, which consequently changes the color of the light produced. Incandescent light gets redder as voltage is diminished, which is comparable to the natural change of sunlight at sunset.
But incandescent lamp-illuminated projectors are only satisfactory for illumination of small areas.
In practice, in very large halls such as theatres or where outdoor or daylight presentations must be made, high intensity illumination sources must be utilized.
Originally developed for outdoor and industrial applications, high intensity discharge (HID) lamps are also used in office, retail, and other indoor applications.
There are several advantages to HID sources:
relatively long life (5,000 to 24,000+ hrs),
relatively high lumen output per watt,
relatively small in physical size.
However, the following operating limitations must also be considered. First, HID lamps require time to warm up. It varies from lamp to lamp, but the average warm-up time is 2 to 6 minutes. Second, HID lamps have a “restrike” time, meaning a momentary interruption of current or a voltage drop too low to maintain the arc will extinguish the lamp. At that point, the gases inside the lamp are too hot to ionize, and time is needed for the gases to cool and pressure to drop before the arc will restrike. This process of restriking takes between 5 and 15 minutes, depending on which HID source is being used. Therefore, good applications of HID lamps are areas where lamps are not switched on and off intermittently.
Particularly, such illumination systems cannot be dimmed by direct voltage control of the lamp itself since a reduction in current supply to the lamp results in the extinguishing of the lamp. Control of the light intensity of such lamps can only be achieved through a mechanically working dimming device in the light path. Such light valves have taken the form of an iris, similar to that used in cameras, or a set of shutters or flaps likewise adjustable to permit intensity control adjustment by adjusting the opening to pass more or less light according to the needs of the user.
For this purpose, it has been proposed, for instance in U.S. Pat. No. 5,904,417 A, that a lighting apparatus includes a discharge arc lamp illumination source, an elliptical reflector completely surrounding the illumination source for collecting and projecting light from the illumination source, a shutter for dimming the amount of light generated by the illumination source and emanating from the lighting apparatus. The shutter is mechanically operable to define an optical passageway there through of variable cross sectional area. The shutter includes a plurality of shutter blades mounted in a movable relationship relative to each other in response to user input. The shutter blades is movable to define the optical passageway of variable cross sectional area to dim the amount of light from the illumination source passing there through over the range of from about 0 to 100%.